This invention relates generally to air cushion vehicles and more particularly to a vibration suppressing means for an aircraft installed air cushion type takeoff and landing system having an inflatable trunk configuration. Conceptually the use of an air cushion to support a vehicle above the ground is well known in the technology. Examples of classical approaches to confining a pocket of air beneath vehicles are disclosed in U.S. Pat Nos. 3,172,494; 3,208,543; 3,283,355; 3,297,102; 3,353,617; 3,400,779; 3,877,542 and 3,891,047.
One recent refinement of the air cushion technique consists of using a structure generally referred to as a trunk to provide both the air duct and seal for the air cushion. The trunk is a large inflatable tube encircling the bottom of the vehicle, having at its lowest surface multiple air passage holes through which pressurized air from inside the tube both supplies the cavity formed and acts directly on the ground beneath. The air used to inflate the trunk and eventually pressurize the cavity is normally supplied by a motor driven fan.
The use of an inflatable trunk to perform the function of both an air duct and a seal is especially important in aircraft applications as a takeoff and landing system. Prior to the development of such trunks, substantial power requirements, approaching that of a vertical takeoff type aircraft, were necessary to provide a practical hoverheight distance between the ground and the aircraft's hard structure. In fact, this was the major shortcoming in the development of early air cushion type takeoff and landing systems.
In a typical aircraft application the trunk is deflated, and thereby retracted to the aircraft fuselage, during normal flight to reduce aerodynamic drag. The trunk is inflated for takeoff and landing, by introducing air from a compressing fan, to obtain the benefits of air cushion flotation, such as low ground overpressure. Using the inflatable trunk approach to air cushion support provides an aircraft hoverheight adequate to transverse most obstacles, in cases up to 2/3 of the trunk depth, without increasing the daylight clearance or lift power.
During the recent evaluation of such a trunk type air cushion takeoff and landing system, a number of aero-mechanical performance degrading phenomena were encountered. The invention addresses itself to the correction of one such problem, generally categorized as a vertical vibration or flutter of the trunk structure appearing when the air cushion supported aircraft travels at ground level over any smooth hard surface, such as a runway. Unsuppressed vibrations significantly reduce the operational life of the flexible trunk, causing surface splitting and internal delamination.
A variety of techniques were attempted to suppress or measurably dampen the trunk vibrations. With the exception of the invention disclosed herein, they proved to be of limited success. Mounting rubber strakes along the outer lateral edges of the trunk bottom, attaching mass type damper weights to the trunk sides, installing diaphragms internal to the trunk tube, and shifting of the aircraft's center of gravity describe the significant techniques considered and tested. Though some did suppress or moderate vibrations under limited conditions, detrimental secondary effects, such as wear damage to the trunk, unstable heaving of the aircraft and constraints on trunk pressure, preclude their operational implementation as solutions to the vibration problem.